The present invention relates to an IC card for exchanging data with a data reading and writing device.
There are two types of IC cards for exchanging data with a data reading and writing device: contact cards, which exchange data through an electrical connection with the data reading and writing device, and contactless cards, which use radio frequency (RF) signals or other non-contact media to exchange data with the data reading and writing device. FIG. 7 is a block diagram of an exemplary conventional contactless IC card.
As shown in FIG. 7, this noncontact IC card 100 (simply IC card below) communicates with a data reading and writing device (reader/writer) 101 via an antenna circuit 102 enabling RF communications, and a modulation/demodulation circuit 103 for demodulating received data and modulating data for sending. An input/output (I/O) circuit 104 converts serial data to parallel data, and parallel data to serial data. The memory unit 107 of the IC card 100 comprises a sense amplifier unit 106, and non-volatile memory 105 such as an EEPROM unit. A control circuit 108 controls the operation of the I/O circuit 104 and memory unit 107, and an internal power supply circuit 109 rectifies the RF signal received by the antenna circuit 102 to supply power to the other internal circuit components. A bus 110 connects the I/O circuit 104, memory unit 107, and control circuit 108.
FIG. 8 is a circuit diagram of a sense amplifier used in the sense amplifier unit 106, and is described in detail in Japanese Patent Application HEI 7-1304 (1995-1304) filed by the inventors of the present invention. What is important to note here is that the speed of charging the parasitic capacitance 123 of the bit line 122 connected to the memory cell 121 of the non-volatile memory 105 is different when a charge is stored or is not stored in the memory cell 121.
A capacitor 124 with a specific capacitance is therefore provided to detect the presence of a charge stored in the memory cell 121 by comparing the charging rate of the parasitic capacitance 123 and the capacitor 124 to the current flowing from nMOS transistors 126 and 127 of the current supply unit 125 during reading. More specifically, the capacitor 124 is used to detect and read the binary level of the data stored in the memory cell 121 during reading. It should be noted that when the non-volatile memory 105 has an 8-bit structure, the sense amplifier unit 106 has eight of the sense amplifier circuits shown in FIG. 8, one sense amplifier circuit corresponding to each bit, and that just one of these is shown in FIG. 8.
When the non-volatile memory 105 of this IC card 100 is accessed by a reader/writer 101, the control circuit 108 compares the password input from the reader/writer 101 with the password previously stored in the non-volatile memory 105. When the passwords match, the control circuit 108 permits access to the non-volatile memory 105 according to the commands input from the reader/writer 101. When the passwords do not match, however, the control circuit 108 prohibits access to the non-volatile memory 105, and implements a particular error processing routine. As a result, the reader/writer 101 cannot access the non-volatile memory 105 if the passwords do not match.
The operating voltage range of the sense amplifier unit 106, however, is narrower than that of the other circuit components. More specifically, the lowest operating voltage of the sense amplifier unit 106 is higher than that of the other circuit components. This means that if the supply voltage from the internal power supply circuit 109 drops, the sense amplifier unit 106 may stop operating while the other components continue to operate. The output from the sense amplifier unit 106 in this case will be HIGH, and, if the non-volatile memory 105 has an 8-bit structure, the output of each sense amplifier circuit in the sense amplifier unit 106 will be HIGH.
If a password from the reader/writer 101 is checked while the sense amplifier unit 106 is in this non-operating state, the password will be verified (FF) a determination of whether or not the password matches the password stored in the non-volatile memory 105, because all outputs from the sense amplifier unit 106 are HIGH.
The power source of the IC card 100 is derived by rectification of the RF signal from an external source by the internal power supply circuit 109. As a result, the internal supply voltage of the IC card can be easily changed by simply moving the IC card 100 closer to or away from the reader/writer 101. This means that if the IC card 100 is moved to a position lowering the supply voltage to a level at which only the sense amplifier unit 106 stops operating, password verification is then completed with a potentially false verification (FF). When the IC card 100 is then moved to a position at which the supply voltage is again raised to a level at which the sense amplifier unit 106 operates, the non-volatile memory 105 can be accessed even though the password has not been properly verified. This obviously means that it is possible to access the non-volatile memory 105 even without knowing the correct password, and data security cannot, therefore, be assured.
This problem is not unique to contactless IC cards. The same type of problem also occurs in contact IC cards when the supply voltage to the IC card is dropped to a level at which only the sense amplifier unit of the memory unit does not operate.
Therefore, an object of the present invention is to provide an IC card resolving the above problem by preventing data access when the password is not confirmed to have been properly verified, and, thereby, improving the security of the data stored in memory.